Charles and David Koch are each worth about $25bn, which makes them the fourth richest Americans. When you combine their fortunes, they are the third wealthiest people in the world. Radical libertarians who use their money to oppose government and virtually all regulation as interference with the free market, the Kochs are in a class of their own as players on the American political stage. Their web of influence in the US stretches from state capitals to the halls of congress in Washington DC.